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We analysed and compared the effects of grouping vs. isolation on female reproduction in Helleria brevicornis, particularly with regard to the formation of marsupia and the production of offspring. 1. The percentage of females which developed a marsupium during the reproductive period was found to be significantly lower among isolated females than among females which were reared in groups. This decreased percentage was found to be partly due to the fact that isolated females developed incomplete marsupia, and partly to the belated occurrence of moult in these females (by lengthening of the duration of intermoult). Belated moults do not involve development of a marsupium when they occur after the end of the reproductive period. 2. Offspring production was also found to be lower in isolated females than in those studied under group conditions. However, this drop in productivity only took place at the second period of reproduction. 3. No group effect was found during the juvenile period; and even the first reproductive period was not affected, whether females had been isolated or reared in groups as juveniles. Group conditions had repercussions only in females which had reached the adult stage. The heterogeneity observed among Helleria as regards the sensitivity of juveniles and adults to group conditions is discussed, from the functional point of view, as possibly playing a role in population dynamics.